You can have two pieces of fabric that appear to fit perfectly together when they’re lying on the table before you sew. Yet, when you’re finished sewing them together, your project can be slightly skewed or uneven. The culprit isn’t always incorrect cutting, thread tension, a machine malfunction, or the quality of the thread. It could just be that one side of your seam line was a bit too far from the cut fabric edge while the other side was a bit too close to the cut fabric edge. That difference can affect the final size of your project, how well corners align, and how flat it presses at the end.
When you cut a fabric pattern, there will often be directions for “seam allowance” which means the measurement from the edge of fabric to the actual line of stitching. It might be written as a number on the pattern or in your beginner sewing class, it could be the distance of the stitch line relative to one guide line on the needle plate. Seam allowance is an important consideration because it’s the extra room you need so that your seam can be stable; it helps to join pieces at the intended sizes and it allows you to press or finish the raw edge of the fabric when appropriate. When you allow that measurement to grow or shrink as you move through your stitching line, you could run into problems with your sewing project getting “out of whack” a bit.
The importance of seam allowance applies to some very simple sewing projects, like a small pouch or a hem. For example, one side of the side seam of a small pouch may take up a bit more seam allowance, resulting in the pouch ending up skewed or not even at all. When sewing a hem, if it’s not measured the same amount of allowance as you go, it’s likely your fabric will turn out looking like a twisted ribbon as the beginning measurement of your seam allowance differs from the ending measurement. The notches on a pattern may not match up at the very end of the stitching because you were just a bit further away or closer to the fabric edge. Yes, you will have sewn the two pieces of fabric together, but your construction will feel more cumbersome and more difficult than the pattern or project requires it to be.
Use a piece of scrap cotton fabric to experiment with this concept. Take two pieces of the same fabric and the same size. Draw your seam lines on them with chalk and a ruler. For one sample, sew your seam with the fabric edge parallel to your needle plate line and your other hand following along to hold the fabric. For the other sample, sew the fabric closer and further away from the edge, alternating on purpose. Press the seam allowance to one side and compare the finished samples. The uneven sample will reveal a reason why you want to keep your seam allowance even to help you avoid a bit of trouble later on.
It’s helpful to look at your fabric edge, not just the stitching. The needle tells you where the stitching is going, and the guide on your machine helps you determine whether the fabric is the proper distance from the actual stitch line. Maintain a firm grasp on the fabric to keep it from straying as the machine feed dogs move it through the machine. If necessary, correct as you go, but do so gently. Should your fabric be cut unevenly, follow the approximate line instead of trying to adjust too quickly with every stitch or so.
Using pins or clips can have an impact on your seam allowance as well. If you pin layers out of place so that the bottom layer gets pushed out while the top one doesn’t, your seam allowance will be affected. It’s important to use a pinning method that keeps the fabric flat so that you can easily see your seam line and your edge as you work. With more complex projects, especially curved seams, your seam allowance requires even more attention because the edge is curved but the stitching line needs to stay more even. Take small pauses with the needle down and gently turn as you go to avoid stretching the fabric.
Before you proceed to stitch a seam, think back to that original question: where should the stitching line be? Review your instructions from the pattern, the seam line marked on the fabric, and the needle plate guide as needed before you press the foot pedal. By ensuring that your seam allowance is consistent throughout, you make later steps easier to execute and feel more confident in the process. Corners will align better and be easier to sew in. Hems will fold more easily and smoothly. Pressing will help you visualize the construction of your project rather than masking the tiny irregularities.